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Abstract

1. Harbour seals Phoca vitulina in eastern England were heavily exploited in the 1960s and 1970s, and affected by phocine distemper virus (PDV) epidemics in 1988 and 2002. Information on their historical and current status is required for their management. To maximize the effectiveness of limited population survey effort we need to both estimate and minimize error.

2. Presented here are data from annual aerial surveys of the population. Sporadic, synoptic surveys in The Wash, England, were used with more frequent counts of subpopulations in the Moray Firth, Scotland, to determine optimum timing of surveys.

3. We developed models that explicitly account for variability in both observation and population growth processes to show that the proportion of animals observed is much more variable than the annual growth rates. The latter can therefore be treated as constant within each period, and estimated along with the epidemic mortalities during the study and the precision of the survey results.

4. The Wash population increased at around 3.1% per annum (pa) [95% confidence interval (CI) 2.1-4.1] between 1973 and 1988. It fell by approximately 52% (95% CI 44-59) as a result of the 1988 PDV epidemic, and subsequently increased at 5.7% pa (95% CI 4.8-6.7). These growth rates were below those reported for European mainland populations, but showed no indication of density-dependent effects.

5. The recurrence of PDV in 2002 caused approximately 22% mortality (95% CI 9-33), significantly less than the 1988 epidemic and less than half that in European mainland populations in 2002.

6. Synthesis and applications. Combining sparse, systematic survey data with sporadic counts produced robust estimates of growth rates and epidemic mortality. The results indicate the value of even limited and sporadic survey effort for monitoring populations. The study has highlighted significant differences in both population dynamics and the severity of disease events between English and European harbour seal populations.